Alpha output without background

Hi everyone,

I was wondering if there’s a way to output pure alpha channel renders to a PNG image in a way that ignores the colour of the background. I’m trying to make some animated sprites of explosions, and I’m rendering against a black background, so when the explosion becomes thin around the edges, the output colour is mixed with the background.As a result, I get an ugly, smudgy black line around the edge of the sprite when I put it against, say, a white background. Any way to solve this?

In 2.49 there are three buttons called “Sky, Premul, and Key”. Sky is the default, but you probably want Key. Press F10, then click the “Key” button and render a test.

Hmmm… key gives me the right colours, but with 2-tone alpha.

Premul gives me the right alpha, but still makes transparent areas darker.

I need some way to composite the output - with the colour from Key and the alpha from Premul.